1. The Field of Invention
The present invention is generally directed to methods and apparatus for the optical recording of data. More particularly, the present invention relates to optical recording methods and apparatus utilizing materials having the property of birefringence.
2. The Prior Art
With the constant improvements in the field of computer technology, it has become necessary to improve the means by which data can be rapidly and accurately recorded and accessed. Furthermore, the increasing dependence upon computer generated and recorded information and data has created problems in storing the vast amounts of information and data which are now available. As a result, those skilled in the art have continued their search for recording media which are capable of high density data storage, and yet are amenable to high-speed writing and reading processes.
Additionally, in recent years, the development of the entertainment industry has led to a need for improved methods and media for storing audiovisual performances and the like. Indeed, a substantial market is developing around the sale and lease of entertainment-oriented videodisk and videotape products, thereby accelerating the investigation into such improved storage media.
Storage media are typically categorized into four general classifications: permanent storage media (on which information is recorded and then the media is rendered incapable of further recordation), permanent archival storage media (which is similar to permanent storage media except that it has a much longer lifetime), postable storage media (information can be subsequently recorded on the media after the initial recordation), and erasable media. It will be readily appreciated that with the ever expanding uses for storing data (including audiovisual presentations), the need for new and improved recording methods and media in each of these classifications is similarly increasing.
While an erasable medium may be considered to be the most versatile, that is not always the case. For example, with record keeping and information retrieval systems, erasability is rarely needed, and is generally even considered undesirable. In addition, for those uses where it is anticipated that a record may need to be updated, a postable storage medium is typically used so that a code may be added to the recorded information leading to the corrected record. In this way, the latest record is easily accessible, but the previous record is also available.
In the past, magnetic tapes or disks have been the common storage media for information and data. While such magnetic storage means have the advantage of being erasable, they also have the disadvantage of being relatively low-density data storage media. Accordingly, a great deal of attention has been directed towards the development of optical recording media, which are capable of a much higher density recording of information. Because most optical recording methods and media are not erasable, they have generally been used for archival storage of records, documents, music, pictures, motion pictures, or other information.
In general, optical recording media employ a focused laser to induce a chemical or physical change at the point of contact on the recording medium, thereby forming a "spot" about one micron in size. In order to "write" information, the information is first converted into a digital format. For instance, when utilizing a binary format, the information is converted into a format having two symbols, e.g., "on-off", "black-white", or "yes-no". By coupling electrical impulses having a binary format to a light beam modulator, it is possible to reproduce the "digitized" information as a set of light and dark spots on a photosensitive medium.
In order to "read" the information, the medium is scanned by a focused laser at low power, and the pattern of light and dark spots is observed to reconstruct the binary code, which in turn can be readily converted back to its original form.
Currently, eleven different classes of optical recording materials have been proven or are presently considered to be potentially useful (to varying degrees) in recording information: photographic films, photoresists, photopolymers, thermoplastics, photochromics, chalcogenide films, ablative thin films, magneto-optic, photoferroelectric, photoconductive/electro-optic, and electro-optic. Each of these are hereinafter discussed briefly.
Photographic films are prepared by placing a light-sensitive silver halide emulsion onto a rigid or flexible substrate. The recording process is photochemical in nature and results in an optical density change within the emulsion. A processing step, which is necessary before the data can be read, results in the appearance of a series of light and dark spots on the photographic films. The sensitivity and density of data storage on such photographic media are necessarily dependent upon and limited by the grain of the film.
The fact that photographic films must be processed after the information is recorded and prior to reading the information is a significant disadvantage. Not only does this additional processing step require additional time and facilities, but it also makes it impossible to read the information immediately after writing. It is considered very advantageous to be able to read immediately after writing because it is then possible to verify that the information was recorded correctly. Another disadvantage of photographic films is the fact that they have relatively low sensitivity. As a result, relatively long laser pulse durations are required for the writing step, thereby making the writing process relatively slow when these materials are used. Also, the relatively high costs of the fabrication of photographic films (especially of the extremely fine-grain films), as well as the additional development costs, militate against their use.
Photoresists are light-sensitive organic materials which, upon exposure and development, form image relief patterns. The readout process measures the difference in phase between light reflected from the the relief patterns in comparison to light reflected from the unexposed areas lacking the relief patterns. Again, a processing step is required prior to reading the relief patterns formed by the photoresists. Unfortunately, these materials are unstable to heat or light and, before recording, must be maintained in total darkness at very low temperatures. Moreover, these materials have a low sensitivity and thus require a slower writing process. As a result, these materials are currently unacceptable as a practical recording media.
Photopolymers are organic compounds capable of undergoing photochemical reactions when irradiated with light having a certain frequency. These photochemical reactions result in products having refractive indices substantially different from those of the starting material. Thus, the recorded data are read by observing the presence or absence of localized changes of the refractive index of the medium. A major difficulty with these materials is that if reading is done with the same laser which is used for "writing," even at every low power, additional writing on the recording medium will occur. Thus, for any practical use of such photopolymers, it is necessary to use a second laser wavelength for reading at a frequency that is not absorbed by the recording medium; it will be appreciated that the need for such a second laser wavelength substantially limits the versatility of such a system. Again, like many of the other systems, relatively low sensitivity is a problem with most photopolymers on which tests have been conducted.
Ablative thin films are currently the most widely used media for optically recording data. Such media utilize a thin film of a material capable of absorbing light at the desired frequency which is coated onto a substrate. The recording mechanism is essentially thermal in nature and utilizes the energy of an absorbed laser beam to either melt or actually ablate the material; the result is that dips or even holes are formed in the film. Readout can be either by means of reflection or transmission of light wherein the ablated holes or dips are used to modulate the intensity of a light beam.
Currently, the preferred material for use in these ablative thin films is tellurium. However, even tellurium is disadvantageous because of its high cost, and its instability in air over long periods of time. Moreover, the process by which a tellurium thin film is deposited on a substrate (such as a disk) is very complicated and time consuming; also, the process of forming a uniform thin film is very difficult. A major difficulty with all ablative thin films to data (although it has been minimized by use of tellurium) is the need for relatively large amounts of laser energy for writing.
Thermoplastic materials are prepared by forming a multi-layer structure consisting of a substrate, such as glass or mylar; a thin conductive layer, such as gold or silver; a photoconductor, such as polyvinylcarbazole sensitized with trinitro-9-fluorenone; and a thermoplastic. The recording technique consists of forming a uniform charge on the surface of the thermoplastic so that the voltage is capacitively divided between the photoconductor and the thermoplastic layers. Upon optical exposure, the photoconductor conducts at the points of illumination and thus discharges the voltage at that point. After exposure, the thermoplastic is heated so that the electrostatic forces deform the surface of the thermoplastic into a relief pattern which corresponds to the optically recorded information.
The advantage of using thermoplastic materials is that the information may be erased by additional heating so that surface tension smooths out the relief pattern. Reading is done in a manner similar to that used in connection with photoresists. However, these materials are disadvantageous because it has not yet been possible to achieve high-density recording of information and the materials developed to date have been prohibitively expensive.
A photochromic material is one which may exist in two or more relatively stable states having different optical properties and which may be switched from one state to the other by photon radiation. This change of state may result in either different absorption spectra or a difference in refractive index. This medium presently suffers from the problem of only being able to achieve a low sensitivity. These materials also suffer from the same disadvantages mentioned above in connection with photopolymers.
Chalcogenide materials reversibly switch between the amorphous and crystalline states upon heating, such as with a laser. Hence, data can be recorded in binary format by changing the state of the chalcogenide materials. Readout is accomplished by measuring differences in either reflection or transmission between the crystalline and amorphous states. The chalcogenides are of interest because they open up the possibility for erasure and recycling. However, they are currently of little practical significance because they are somewhat unstable, require relatively high laser power for writing, must be formed as a thin layer in a manner similar to the ablative thin films, and must be read with a second laser having a different wavelength than the writing laser.
Magneto-optic materials utilize differences in directions of magnetization to store information. A temperature rise in a localized area resulting from a focused layer reverses the local direction of magnetization. Readout is accomplished by utilizing either the Faraday effect for transmission readout or the Kerr effect for reflective readout. Again, this medium is one which may prove useful where erasure of the stored data is desired, although it has not yet been demonstrated to be workable.
A photoferroelectric medium is prepared by forming a photoconductor-ferroelectric sandwich. The photoconductor absorbs the light from a focused laser and induces switching between stable states of the ferroelectric material. In order to read stored information, it is necessary to utilize polarized light and crossed polarizers. The major drawback to the use of these materials has been the need to maintain an electric field to prevent erasing. Accordingly, these materials are impractical for use in any type of long-term informational storage.
Electro-optic materials rely upon the generation of electric fields by means of photoexcitation with resulting spatial rearrangement of electrons, which thus causes changes in the refractive index of the material. As discussed above, these localized changes in the refractive index can be "read." Unfortunately, it has not yet been demonstrated that these materials have practical application.
A photoconductive, electro-optic material is one that is both photoconductive and also exhibits a linear electro-optic effect. Such a material is sandwiched between insulating layers, and a voltage difference is maintained on the two insulating layers. Upon exposure, a redistribution of electric charge causes a retardation of light passing through the media. This retardation can be detected by the use of polarized light and crossed polarizers. Although such materials may be reused indefinitely, image storage with these devices is, unfortunately, limited to only a few hours.
From the foregoing, it can be seen that numerous methods have been devised or theorized for using a focused laser to induce some type of a localized change in a recording medium. The presence or absence of change at a given position on the medium serves as a binary code indicator. Conventional equipment is readily available to detect differences in the recording medium, whether those differences result in changes in optical density, refractive index, absorption spectra, magnetic alignment, phase retardation of light waves, and the like.
Several criteria are desirable in evaluating a particular medium. As mentioned above, ablative thin films are presently the optical recording medium of choice among the various types of optical recording media because, in comparison to the other types of media, ablative thin films do not require processing, have relatively high signal to noise ratios, and have relatively high optical absorption at the writing wavelength. Unfortunately, such ablative thin films are expensive to fabricate, require relatively high writing energy, and are not stable for long-term storage in an oxygen atmosphere.
Accordingly, it would be a significant advancement in the art of optical recording if a medium could be provided that utilizes a low writing energy, does not require processing, has a high signal to noise ratio and low bit error rate of written spots, has a long lifetime, and has low fabrication costs. It would also be extremely advantageous if a medium having these features could be read at the same wavelength used for writing.